1. Field of the Invention
The invention relates to a nasal mask for use with an intermittent positive pressure ventilation system for introducing a therapeutic gas, such as oxygen or nitrogen monoxide into the lungs of a patient.
2. Description of the Related Art
An intermittent positive pressure ventilation system is generally used for supporting a ventilatory insufficiency patient. FIG. 8 shows a nasal mask 100 of a prior art which includes a mask shell 102 and a flexible cushion 104 which is detachably attached to the mask shell 102. The mask shell 102 is fluidly connected to an air source 112 through an air conduit 106. The air source supplies air to the mask 100 alternately at high pressure, for example at 20 cm H2O, referred to inhalation pressure, and at low pressure, for example at 4 cm H2O, referred to exhalation pressure. An exhaust port 110 is provided in the air conduit 106 for discharging the exhalation gas from the patient using this nasal mask 100.
A therapeutic gas inlet port 108 is provided in the mask shell 102 which is fluidly connected to a therapeutic gas source (not shown) by a tube (not shown).
The therapeutic gas inlet port 108 is oriented to a region Rn within the nasal mask 100 under the nose of the patient using this nasal mask 100.
With the nasal mask 100 of FIG. 8, a large portion of the inside volume of the nasal mask 100, except for the region Rn under the nose of the patient, is filled with the exhalation gas from the patient, which includes carbon dioxide gas at high concentration, just before the initiation of the inhalation phase of the next breathing cycle because the exhaust port 110 is provided on the air conduit 106 away from the nose of the patient. Therefore, the patient will draw the exhaled gas in again. This increases the carbon dioxide concentration in the blood of the patient using the nasal mask 100 of the prior art.
On the other hand, in the region Rn under the nose within the nasal mask 100, an oxygen rich space is generated just before the initiation of the inhalation phase of the next breathing cycle, since the oxygen gas inlet port 108 is oriented to the region Rn and the flow rate of the exhalation gas from the nose decrease substantially to zero at the end of the exhalation phase. The volume of the oxygen rich space depends on the magnitude of the breathing and the higher the magnitude, the smaller the volume of the oxygen rich space produced in the region Rn under the nose. The volume of the oxygen rich space also depends on the flow rate of the oxygen gas supplied through the therapeutic gas inlet port 108 and the higher the flow rate, the larger the volume of the oxygen rich space produced. This will be a problem when the patient sleeps because the patient will draw an inhalation gas of high oxygen concentration, which results in depression of the respiratory center to further decrease the breathing.
Another nasal mask is disclosed in WO 98/34665 which includes an exhaust port provided in the mask shell and a therapeutic gas inlet port provided on the air conduit for supplying air. The configuration of the nasal mask can substantially avoid or reduce the above-described problems. However, the nasal mask of WO 98/34665 involves another problem that the most of the oxygen gas supplied to the mask during the exhalation phase is entrained into the air flow through the air conduit and discharged through the exhaust port without being used. Further, with the nasal mask of WO 98/34665, a very large amount of oxygen is required in order to increase the peak of the oxygen concentration in the inhalation gas which the patient draws during the inhalation phase.
The invention is directed to solve the prior art problems, and to provide an improved nasal mask which provides a high peak therapeutic gas concentration in the inhalation gas and reduces carbon dioxide rebreathing.
According to the invention there is provided a nasal mask, for use with a system for supplying air with a therapeutic gas to the airways of a patient, which includes a mask shell which is adapted to be put over the nose of the patient using the nasal mask; an air inlet port through which air is supplied from an air source; an exhaust port, provided in the mask shell, for discharging the exhalation gas to atmosphere; and a therapeutic gas inlet port, provided on the mask shell adjacent to the exhaust port. The therapeutic gas port is fluidly connected to a therapeutic gas source which supplies a therapeutic gas at a constant flow rate. The therapeutic gas port is oriented toward a portion of the inside volume of the nasal mask over the nose of the face of the patient using the nasal mask.